1. Field of the Invention
This invention relates to a mechanical seal for sealing a shaft with respect to a housing or the like from which it extends and is customarily employed in connection with rotary machines such as centrifugal pumps, turbines, and the like. An example of a seal for this general purpose is disclosed in U.S. Pat. No. 3,198,530 issued to E. W. Conklin on Aug. 3, 1965.
In general this type of seal employs a stationary sealing member known as an insert, carried by the wall and surrounding the shaft. The insert provides a radially extending planar sealing surface relative to the axis of the shaft. Another member which is rigidly secured to the shaft, herein called an anchoring member, surrounds the shaft at a short distance from the insert and rotates with the shaft without any longitudinal movement on the shaft. A third member, herein called a rotary sealing member, is telescopingly engaged with the member anchored on the shaft and has an end surface in the form of a planar sealing surface. The third member is adapted to move endwise relative to the anchoring member until it is in sealing engagement with the sealing surface on the insert. This sealing engagement provides the primary seal between stationary and rotating parts.
The last mentioned sealing member may be moved longitudinally into sealing engagement by means of spring means or the like, or by means of the pressure sealed against, or by a combination thereof. It is intended to be held in alignment with the shaft by telescoping engagement with the member anchored on the shaft, and means is provided between the two members for driving the third member to rotate with the shaft. Separate seals, herein may be called secondary seals, may be provided between the member anchored on the shaft and the shaft itself and between the member anchored on the shaft and the third member which moves longitudinally relative thereto into sealing engagement. Since there is no movement between the anchored member and the shaft, the secondary seal between them is called a "static" seal. There being a slight axial movement between the anchored and movable members, the secondary seal between them is called a "dynamic" seal.
2. Description of the Prior Art
Various types of seals have been employed between the member rigid on the shaft and the shaft itself on the one hand and between such member rigid on the shaft and the movable sealing member telescopingly engaged therewith. One example of such a seal is shown in the Conklin U.S. Pat. No. 3,198,530 above cited. However, in the case of the Conklin patent the seal between the anchored member and the shaft and that between the anchored member and the movable member are the same and in effect the seal is provided directly between the shaft and the movable member.
The problem of providing an adequate seal with the V-type sealing members illustrated in this patent is several-fold, but perhaps most importantly lies in the fact that in order to make such seals effective it is essential to spread the lips of the seal with such force as to induce a considerable frictional engagement with the movable sealing member and hence to prevent it from being moved freely into sealing engagement with the insert. It is essential that the movable member be movable freely so that a means providing a specified sealing force may be fully effective at all times in maintaining a predetermined desired sealing pressure between the two sealing members. Excessive frictional resistance by high friction type of sealing rings or the like may serve to substantially reduce the sealing pressure between the two sealing members and cause them to fail to maintain an adequate seal therebetween. Furthermore, during operation, if, as almost invariably occurs, to some slight degree, the fixed sealing surface plane varies even minutely from a true radial plane, it is necessary for the rotating sealing member to tilt slightly or cock over to a position at a slight angle to a true coaxial position with the shaft, in order to fully seat against the stationary planar surface. Then when the shaft rotates this sealing member must gyrate slightly relative to the shaft. High friction type seal rings, such as the V-type of Conklin, can interfere with such gyration sufficiently to cause excessive wear of the seal surfaces and various other parts of the assembly and cause early seal failure either of the seal ring or of the engagement between the rotating and stationary seal members.
In an effort to provide a seal between the anchoring member rigid on the shaft and the movable sealing member movable relative thereto, as well as between the member rigid on the shaft and the shaft itself, without introducing such excessive friction, the use of O-ring seals in these positions has been practiced. It is well known that O-ring seals introduce a minimum of friction between two relatively movable parts between which they provide what is called a dynamic seal, especially where such movement between the parts is longitudinal. However, certain unexpected difficulties have been encountered in using such O-ring seals as dynamic secondary seals.
One great difficulty encountered is that when a movable part such as the movable sealing member is sealed to another member with which it is in telescoping engagement, with the O-ring surrounding one member and lying within the other, any slight tilting or cocking of the members relative to one another is in no substantial sense resisted by the action of the O-ring, which merely engages the tilting member at diametrically opposite positions and any force it exerts on the tilting member is exerted on opposite ends of a diameter in directions directly opposed to one another. The result is that any cocking or tilting that occurs tends to remain, with the result that the sealing qualities of the O-ring are shortly subjected to deterioration.
Another difficulty with such an O-ring seal as a secondary seal is that the secondary seal is normally fairly closely situated to the primary sealing surface of the movable sealing member where it engages the stationary primary sealing surface of the insert, at which point, under high speeds of rotation, a great deal of heat is generated by the friction between the primary sealing surfaces. This heat causes a great rise in temperature and this heat so generated flows through the material of the movable sealing member and increases very substantially the temperature of the O-ring. The O-rings are necessarily resilient and are normally constructed of a material which will deteriorate much more rapidly under high temperatures than at low temperatures. Also, the rate of deterioration is not directly proportional to the temperature. For example, deterioration at 350.degree. F. might be twice as much as that at 300.degree. F.
It has also been observed that in the case of use of a single O-ring dynamic seal between the anchored member on the shaft and the movable member thereon, an erosion or fretting takes place in the vicinity of the O-ring along the surface of the anchored member. This of course damages the anchored member, and, while the damage may not be of such a critical nature as to cause failure in most cases, it can be the cause of having to work on or replace the anchored member if the erosion becomes too great.
It has also been found that a single O-ring does not provide sufficient friction in a rotary direction to transmit the torque which may be required to rotate the movable sealing member with the shaft, and various mechanical means are provided for this purpose. In some cases, for example, a single ring-like spring is employed for urging the movable sealing member toward the stationary sealing member and this spring will assist the O-ring in transmitting sufficient torque to rotate the movable sealing member.